1. Field
This invention pertains to the field of wavefront measurements, and more particularly to systems and methods of measuring a wavefront of light using a phase diversity wavefront sensor.
2. Description
A number of systems and methods have been developed for measuring a wavefront of light. Such wavefront measurements have been employed in a number of applications, including ophthalmic applications such as measuring aberrations of an eye, and measuring surfaces of objects such as contact lenses.
One wavefront sensor that has been employed in a number of systems for various wavefront sensing applications is the Shack Hartmann wavefront sensor (SHWS). A SHWS includes an array of lenslets which image focal spots onto a detector array. SHWS's have been employed in a variety of ophthalmic and metrological applications.
However, a SHWS has some limitations in certain applications.
For example, with a SHWS, the wavefront is expected to produce a single local tilt. In general, an SHWS has difficulty measuring wavefronts with discontinuities. However, in some applications, and particularly in some ophthalmic applications, the wavefront may have multiple tilts, which may produce multiple focal spots. For example, such discontinuities can be produced by multi-focal optical devices, including multifocal contact lenses and multifocal intraocular lenses (IOL). W. Neil Charman et al., “Can we measure wave aberration in patients with diffractive IOLs?,” 33 JOURNAL OF CATARACT & REFRACTIVE SURGERY No. 11, p. 1997 (November 2007) discusses some problems in using a SHWS to make wavefront measurements of a patient with a diffractive IOL. Charman notes that when a measurement is taken on an eye that has been implanted with a diffractive IOL, the lenslets of the SHWS will produce multiple images and the detector will record multiple overlapping spot patterns. So, it is difficult at best for a SHWS to measure wavefronts produced by multifocal optical elements, such as diffractive IOLs.
Another limitation of the SHWS pertains to its limited dynamic range. For example, to measure ophthalmic aberrations of a human eye over the wide range presented by the human population, as a practical matter one needs to employ an adjustable optical system in conjunction with the SHWS so that operation of the SHWS can be maintained within its dynamic range. This can add to the complexity and cost of the measurement system, and requires alignment that can reduce the measurement precision of the instrument.
Another type of wavefront sensor is a phase diversity wavefront sensor (PDWS), also sometimes referred to as a curvature sensor. A PDWS may be used to analyze wavefronts at two or more planes that are generally orthogonal to the direction of propagation of an optical beam. In general, a PDWS measurement system makes measurements via an optical system that is capable of imaging two or more planes at once, to minimize or eliminate the effects of any time-varying changes in the optical beam. Graves et al. U.S. Pat. No. 6,439,720 describes a measurement system that includes a PDWS. Early PDWS systems employed a relatively complex arrangement of beam splitters and/or optical delays to generate the necessary images.
In 1999, Blanchard, P. B and Greenaway, A. H., “Simultaneous Multi-plane Imaging with a Distorted Diffraction Grating,” APPLIED OPTICS (1999) (“Blanchard”) disclosed the use of a diffractive optical element (DOE) in a PDWS. As disclosed by Blanchard, the DOE uses local displacement of lines in a diffraction grating to introduce arbitrary phase shifts into wavefronts diffracted by the grating into the non-zero orders to create multiple images of the incident light. In Blanchard's arrangement, a diffraction grating having a quadratic displacement function is employed in conjunction with a collocated single lens to alter the optical transfer function associated with each diffraction order such that each order has a different degree of defocus. Greenaway et al. U.S. Pat. No. 6,975,457 and Greenaway et al. U.S. Patent Application Publication 2006/0173328 describe further details of a PDWS that includes a DOE.
Otten III et al. U.S. Pat. No. 7,232,999 discloses the use of a PDWS with a DOE for determining the characteristics of an infrared wavefront produced by a laser. Slimane Djidel, “High Speed, 3-Dimensional, Telecentric Imaging,” 14 OPTICS EXPRESS No. 18 (4 Sep. 2006) describes design, testing and operation of a system for telecentric imaging of dynamic objects with a single lens system. However, the procedure described therein is not extensible to more complicated configurations.
Nevertheless, these references are not generally directed to applications where there is speckle and/or discontinuities or large aberrations in the wavefront, such as may be the case in many ophthalmic applications, including the measurement of IOLs, multifocal contact lenses, etc., and eyes or optical systems that include such devices. Furthermore, these references do not provide a generalized design method for incorporating a PDWS into more complicated optical systems.
It would be desirable to provide an ophthalmic measurement instrument that utilizes the benefits of a PDWS, alone or in conjunction with a SHWS. It would further be desirable to provide such an instrument that can measure wavefronts with speckle and/or discontinuities or large aberrations in the wavefront. More particularly, it would be desirable to provide such an instrument that can perform wavefront measurements for systems that include a multifocal element, such as an intraocular or contact lens that is either a refractive multifocal lens, a diffractive multifocal lens, or a diffractive monofocal lens. It would also be desirable to provide a generalized method of designing a measurement system including a PDWS.
In one aspect of the invention, a phase diversity wavefront sensor comprises: an optical system including at least one optical element for receiving a light beam; a diffractive optical element having a diffractive pattern defining a filter function, the diffractive optical element being arranged to produce, in conjunction with the optical system, images from the light beam associated with at least two diffraction orders; and a detector for detecting the images and outputting image data corresponding to the detected images, wherein the optical system, diffractive optical element, and detector are arranged to provide telecentric, pupil plane images of the light beam.
In another aspect of the invention, a method is provided for measuring a wavefront of an optical system including a multifocal element. The method comprises: providing a light beam to a lens, the lens being a refractive multifocal lens, a diffractive multifocal lens, or a diffractive monofocal lens; directing light from the lens to a phase diversity wavefront sensor, comprising an optical system including at least one optical element for receiving a light beam, and a diffractive optical element the shape of which is defined by a filter function, the diffractive optical element being arranged to produce in conjunction with the optical system images of the light beam associated with at least two diffraction orders; and a detector for detecting the images and outputting image data corresponding to the detected images; and measuring the wavefront of the light from the lens using the image data output by the detector.
In yet another aspect of the invention, a method is provided for measuring a wavefront of an object having first and second surfaces. The method comprises: providing a light beam to the object; directing light from the lens to a phase diversity wavefront sensor, the lens being a refractive multifocal lens, a diffractive multifocal lens, or a diffractive monofocal lens, the phase diversity wavefront sensor comprising an optical system including at least one optical element for receiving a light beam, and a diffractive optical element the shape of which is defined by a filter function, the diffractive optical element being arranged to produce in conjunction with the optical system images of the light beam associated with at least two diffraction orders; and a detector for detecting the images and outputting image data corresponding to the detected images; and simultaneously measuring the first and second surfaces of the object using the image data output by the detector.
In still another aspect of the invention, a method is provided for designing a phase diversity wavefront sensor. The method comprises: providing one or more analytic solutions for paraxial equations that govern an optical configuration of the phase diversity wavefront sensor; providing a set of input design parameters for the phase diversity wavefront sensor; generating a set of output values from the analytical solutions and the input design parameters; and determining whether the output parameters meet a viability threshold.
In a further aspect of the invention, a phase diversity wavefront sensor comprises: an illuminating optical system for delivering light onto a retina of an eye; a receiving optical system for receiving light reflected by the retina, the receiving optical system comprising a diffractive optical element including a diffraction pattern defining a filter function, the diffractive optical element being arranged to produce, in conjunction with the optical system, at least two images from the light beam associated with at least two diffraction orders; a detector for detecting the at least two images; a memory containing instructions for executing a Gerchberg-Saxton phase retrieval algorithm on data produced by the detector in response to the detected images; and a processor configured to execute the Gerchberg-Saxton phase retrieval algorithm so as to characterize a wavefront produced by the reflected light.